A staggered vertical comb drive (SVCD) is a type of MEMS (Micro-Electro Mechanical Systems) actuator capable of relatively high actuator strength and a high speed using electrostatic principal of operation. Furthermore, they can be fabricated using standard materials and scalable processes developed in the semiconductor industry. SVCDs can be advantageously used to control high-speed, high-resolution micromirrors in a variety of optical applications including optical scanning, optical switching, free-space optical communications, optical phased arrays, optical filters, external cavity lasers, adaptive optics and other applications.
The actuation principle of a typical SVCDs is electrostatic. A potential difference is applied between two comb structures, a movable comb, or a rotor, and a stationary comb, or a stator. When a voltage is applied between them, the movable comb (rotor) is drawn toward the stationary comb (stator) until the teeth of the stator and rotor overlap and the electrostatic field energy is minimized. Different types of SVCD devices are described in further detail, for example, in U.S. Pat. No. 6,612,029 to Behin et al, which is incorporated herein by reference.
FIG. 1 illustrates a typical prior art SVCD 20. The SVCD 20 includes a stator 22 and a rotor 30. The stator has individual comb fingers 24 formed on a spine 26. The rotor 30 includes individual comb fingers 32 linked by a spine 34. The rotor 30 also includes a mirror or paddle 40 with associated torsional hinges 42. In a resting state the rotor 30 is positioned substantially above the stator 22 as shown in FIG. 1.
FIG. 2 illustrates the SVCD system 20 in an activated state. This state is achieved by applying a voltage between the rotor 30 and the stationary comb assembly 22. In this state, the individual combs of the rotor and stator interdigitate. The applied voltage attracts the rotor 30 to the stator 22, thus exerting torque on the torsional hinges 42, forcing the mirror 40 to tilt. The torsional hinges 42, which are anchored, provide restoring torque when the voltage is removed.
A typical prior art process flow involves creating the moving comb assembly 30 by etching one silicon-on-insulator (SOI) wafer, and the stationary comb assembly 22 by etching another SOI wafer, and then assembling, for example bonding, the etched wafers together to form the SVCD 20. Different versions of such process are described in U.S. Pat. Nos. 6,925,710, 7,079,299, etc. However, stringent alignment requirements between the two wafers from which the two comb assemblies are formed can considerably complicate the device processing and negatively affect the device yield.
Alignment of the stator and rotor fingers is critical to proper operation of the actuator. Failure to achieve the required alignment can impair the actuator performance and/or reliability as a result of failure modes such as electrical breakdown, mechanical interference, and lateral collapse.
Typically the rotor and stator must be aligned laterally, i.e. in the plane of the wafer and the combs, to approximately one micron or better. However, when the rotor and stator are fabricated from different wafers, accuracy of a front side to back side wafer alignment, and of the lateral alignment of the wafer bond can be of the order of three microns (3 sigma) across a wafer.
To overcome this difficulty, techniques have been proposed for self-aligned manufacturing of the rotor and stator fingers, wherein both the rotor and stator a photolithographically fabricated from a same device layer of a SOI wafer.
For example, U.S. Pat. No. 6,612,029 to Behin, et al discloses a method of simultaneous fabrication of the rotor and stator fingers from the same device layer that includes two conductive silicon layers separated by an isolation layer of a silicon oxide. The final device has at least one set of fingers, for example of the rotor, each of which has two vertically stacked conductive layers separated by the isolation oxide layer. In operation, one of said conductive layers is grounded, and the voltage is applied to the other to create a pulling electrical field between said layer and the adjacent fingers of the stator which are grounded. The stator fingers can be etched down to the isolation layer to form a thinned set of stator fingers. In one embodiment, the isolation oxide layer is removed leaving an air gap in the respective fingers.
Although the SVCD fabrication method disclosed by Behin et al provides self-aligned rotor and stator, it has other disadvantages. One disadvantage of this method is that the multi-layer fingers it forms may suffer from electrical breakdown at high voltages, which effectively limits the applied voltage and thereby—the rotation angle of the rotor. This limitation can be especially severe if the oxide layer separating the conducting layers of the fingers is removed, forming the air gap; additionally, the air gap embodiment can be sensitive to the presence of small dust particles, which can electrically shorten the device. If the oxide layer is not removed, its electrical properties can drift over time; for example, it can accumulate static electrical charge over time altering the electric field coupling the stator and rotor, leading to undesirable variations and/or aging of the device performance.
US Patent application 2007/0026614 to Choo, et al discloses an SVCD fabrication method which is somewhat similar to the method of Behin et al, but wherein the device layer from which the rotor and stator are fabricated in a single conductive silicon layer without the isolating oxide layer in the middle, using a two-layer mask to separately define the rotor and stator fingers. One set of the fingers is thinned by a timed etch process to about half of the device layer height, while the other set remains full-height. Although the resulting device is free from the disadvantages of the Behin SVCD discussed hereinabove, it has others. One disadvantage of the method of Choo et al is that it results in a device with a reduced rotation angle of the rotor, since the applied voltage can only rotate the rotor until a middle point of its fingers is aligned with a middle point of the stator fingers. For example, in the device of Choo, the middle points of the rotor and stator fingers in a rest state, when no voltage is applied, are separated only by about a quarter of the device layer height, as opposed to the separation of about a half of the device layer height for the device of Behin.
An object of the present invention is to provide a self-aligned method of fabrication of SVCD devices that are free from all or at least some of the above described and other disadvantages of the prior art methods.